CBlockTreeDB::ReadReindexing => CBlockTreeDB::IsReindexing

Summary: This is just nicer API.

Test Plan:

ninja all check-all

Reindex a node.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6564